1. Field of the Invention
The present invention generally relates to techniques of multiplexing multiple light waves on an optical transmission medium and in particular to an optical wavelength-multiplexing transmission system allowing high-capacity bidirectional data transmission using a wavelength bandwidth of 100 nm or more per optical transmission medium.
2. Description of the Related Art
For optical high-capacity communication networks using optical transmission lines (typically, optical fibers), Wavelength-Division Multiplexing (WDM) is the current favorite multiplexing technology since multiple WDM channels from different end users can be multiplexed on the same optical fiber. Since an optical fiber has a wide usable bandwidth, it can be divided into a number of non-overlapping wavelength bands, each of which is assigned to a different WDM channel.
A long distance transmission of WDM lightwave signals through an optical fiber requires periodic amplification of the WDM lightwave signals. An optical in-line amplifier such as Erbium-doped fiber amplifier (EDFA) is preferably employed as a repeater amplifier since it can concurrently amplify all the WDM lightwave signals with simplified structure and reduced cost.
A WDM transmission experiment has been recently reported such that 160×20-Gbit/s WDM transmission is successfully made over 1,500 km using optical amplifier repeaters with an optical wavelength band of 64 nm on one-direction signal transmission using both 1.55-μm band and 1.58-μm band where EDFA can amplify lightwave signals (T. Ito et al., “3.2 Tb/s-1,500 km WDM transmission experiment using 64 nm hybrid repeater amplifiers”, Optical Fiber Communication Conference, Postdeadline Papers, PD24).
A necessary condition to improve the capacity of WDM transmission is to widen its wavelength width. However, the amplification-capable wavelength range of a currently available EDFA is limited to 1530-1560 nm and 1570-1620 nm. On the other hand, a silica-base fiber has a usable wavelength range from 1450 nm with relatively low loss. Accordingly, an optical amplification technique providing an operable wavelength range from 1450 nm to 1530 nm becomes important.
There has been proposed an optical amplifier using a thulium-doped fiber that meets the above requirement (see, for example, Japanese Patent Application Unexamined Publication No. 4-66390). A signal light and a excitation light having a wavelength of 1.06 μm are combined to travel within the thulium-doped fiber functioning as a gain medium, resulting in amplification of the signal light at around 1470 nm.
Further, there has been reported a combination of the thulium-doped fiber amplifier and the EDFA in the same direction signal transmission allowing high-capacity transmission at three wavelength bands: 1464-1478 nm; 1535-1558 nm; and 1574-1599 nm (J. Kani at al., “Trinal-wavelength-band WDM transmission over dispersion-shifted fiber”, Electronics Letters, Vol. 35, No. 4, pp. 321-322, 1999). A bidirectional signal transmission may be realized by using two optical fibers, each of which provides two or three wavelength bands according to the above-described techniques. Such techniques allow high-capacity transmission using a wavelength bandwidth of 100 nm or more.
Furthermore, there has been proposed a technique allowing the thulium-doped fiber amplifier to provide the amplification-capable wavelength range from 1480 nm to 1510 nm in which the optical fiber provides lower loss by modifying the pumping method of the thulium-doped fiber amplifier (T. kasamatsu et al., “1.50-μm-band gain-shifted thulium-doped fiber amplifier with 1.05- and 1.56-μm dual-wavelength pumping”, Optics Letters, Vol. 24, No. 23, pp. 1684-1686, 1999). This technique will be used to construct a high-performance broadband transmission system allowing, for example, a longer transmission distance and so on.
In WDM transmission using a wavelength bandwidth of 100 nm or more as a transmission wavelength band, however, Raman scattering within a single fiber becomes significant. Raman scattering is a phenomenon that a scattering of light of a certain wavelength is caused by phonons to produce light of a different wavelength within the fiber. The Raman scattering causes energy to be transferred in part from light of a shorter wavelength to light of a longer wavelength. The amount of energy transferred from the shorter wavelength light to the longer wavelength light is in proportion to the intensity of the longer wavelength light. Accordingly, the higher the intensity of the longer wavelength light, the larger the amount of energy transferred from the shorter wavelength light to the longer wavelength light. In the case of silica-base fiber, it is well known that the amount of energy transferred from the shorter wavelength light to the longer wavelength light is maximized at around 1.5 μm when the difference between the longer wavelength and the shorter wavelength is 100 nm.
In recent WDM transmission using a wavelength bandwidth of 100 μm or more, as described above, Raman scattering causes a shorter wavelength light traveling over a single fiber to be attenuated excessively and, contrarily, the longer wavelength light traveling over the same fiber to be amplified. Hereafter, such a phenomenon is called “inter-band Raman scattering”. When the inter-band Raman scattering occurs strongly, the shorter wavelength light dramatically reduces in power at an output end of the fiber or an input of a repeater, resulting in increased error rate at the output end, deteriorated signal-to-noise ratio, and the like.
It is known that distributed Raman amplification is effectively applied to compensate for excessive loss caused by the Raman scattering. The distributed Raman amplification is caused by Raman scattering effect to amplify light traveling in one direction within the fiber. Specifically, the shorter wavelength light traveling within the fiber in one direction is amplified by the presence of excitation (or pump) light traveling within the same fiber in the opposite direction, the excitation light having a wavelength further shorter than the shorter wavelength light by about 100 nm. In the above WDM transmission, since the maximum amount of energy transferred from the shorter wavelength light to the 100-nm longer wavelength light, by injecting further 100-nm shorter excitation light into the fiber at the output end thereof, the distributed Raman amplification can compensate for power loss of the shorter wavelength light to avoid reduction in signal light power at the output end.
However, in reality, only the distributed Raman amplification cannot sufficiently compensate for signal deterioration caused by attenuation of the shorter wavelength light due to the inner-band Raman scattering. As described before, the amount of energy transferred from the shorter wavelength light to the longer wavelength light is in proportion to the intensity of the longer wavelength light. Accordingly, the shorter wavelength light reduces in power more strongly at a position getting near the input end of the fiber into which the longer wavelength light is injected since the intensity of the longer wavelength light is high at that input end.
On the other hand, the distributed Raman amplification occurs more strongly getting near the output end of the fiber because the excitation light is injected into the output end. However, the shorter wavelength light has significantly reduced in power at the output end due to attenuation caused by the inter-band Raman scattering and attenuation caused by transmission loss while traveling over the fiber. Accordingly, when a very weak light is amplified by the distributed Raman amplification, Signal-to-noise ratio is significantly deteriorated by noise light generated during the amplification process, resulting in deteriorated transmission characteristics and therefore reduced quality of transmission.
In addition, since the longer wavelength light is amplified by the inter-band Raman scattering more strongly getting near the input end of the fiber, the average power of the longer wavelength light becomes high over the fiber. When the longer wavelength light increases in power, nonlinear optical effect within the fiber (for example, nonlinear optical effect due to third-order nonlinear component such as self-phase modulation) also increases, resulting in increased waveform distortion after traveling over the fiber.